An optical head incorporated into an optical recording and reproducing device irradiates laser light through an objective lens on an optical recording medium during the recording and reproduction of signals. A detector incorporated in the optical head detects reflected light from the recording medium through the objective lens. In accordance with this detection, a focus error signal and a tracking error signal are generated. Consequently, the laser light can accurately trace a recording track on the recording medium. A feedback control is executed so as to make these error signals equal zero. Accordingly, the objective lens is moved along a focusing direction and a direction orthogonal to both the focusing direction and track direction (hereinafter referred to as a bias direction), i.e. its position and/or attitude are corrected.
However, when the objective lens is moved along the bias direction to correct a tracking error, the central axis of a light beam from the light source does not pass through the center of the objective lens. Accordingly, the path of the reflected light shifts, which causes the irradiating position of reflected light on the detector to shift.
Thus, even when tracking control is performed accurately, a spurious tracking error signal may be generated. Namely, a conventional optical head presents a drawback in that the tracking error signal may contain both a true tracking error signal and a spurious tracking error signal which corresponds to the displacement of the objective lens in the bias direction.
In order to solve such a problem, for example, a light spot position detector that detects the degree of displacement of objective lens, disclosed in Japanese Publication for Unexamined Patent No. 1989-79943, is known. As shown in FIG. 5, a light source 22 (or the light source 22 reflected in a mirror) is placed in a position opposite to a four-quadrant light receiving element 21. A light spot 23 is projected from the light source 22 onto the light receiving element 21. Although not shown in the figure, the light source 22 is designed to move together with the objective lens. Moreover, when the central axis of the light beam emitted from the light source of the optical head passes through the center of the objective lens, the center of the light spot 23 coincides with the intersection of two dividing lines of the light receiving element 21 (hereinafter referred to as the central dividing point).
Detection currents Ia, Ib, Ic and Id sent from quadrant light receiving elements 21a, 21b, 21c and 21d are respectively converted into voltages Va, Vb, Vc and Vd by amplification in a current-to-voltage conversion circuit 24. The current-to-voltage conversion circuit 24 is constituted by four sets of operational amplifiers respectively connected to the quadrant light receiving elements 21a to 21d and feedback resistors Ra, Rb, Rc and Rd. The converted voltages Va to Vd are determined by an arithmetic circuit 25 installed in the next stage such that the position of the light spot 23 can give information about the position of the objective lens in the bias direction and focusing direction. As a result, an X-direction output and a Y-direction output are transmitted. In FIG. 5, the X direction corresponds to the displacement of the objective lens in the bias direction, and the Y direction the displacement of the objective lens in the focusing direction.
For example, when the light source 22 moves along the X direction, the amounts of light received by the quadrant light receiving elements 21a and 21d increase (or decrease) while the amounts of light received by the quadrant light receiving elements 21b and 21c decrease (or increase). Therefore, the degree of displacement of the objective lens in the bias direction can be detected by the equation: EQU X-direction output=(Va+Vd)-(Vb+Vc)
Similarly, when the light source 22 moves along the Y direction, the degree of displacement of the objective lens in the focussing direction can be detected by the equation: EQU Y-direction output=(Va+Vb)-(Vc+Vd)
According to this theory, when the position of the light spot 23 on the four-quadrant light receiving element 21 in the X direction and in the Y direction is simultaneously detected, a position signal in the bias direction and a position signal in the focusing direction with respect to the objective lens can be detected. Additionally, the spurious tracking error signal can be cancelled by subtracting the position signal in the bias direction from the above-mentioned tracking error signal. This permits only the true tracking error signal to be transmitted, and therefore the objective lens can be controlled more accurately.
With this configuration, however, the gains with respect to the respective outputs from the quadrant light receiving elements 21a to 21d are fixed by the feedback resistors Ra to Rd in the current-to-voltage conversion circuit 24. Therefore, crosstalk, to be described later, occurs between the X-direction output and the Y-direction output of the arithmetic circuit 25. The crosstalk deteriorates the accuracy of the detection of the position of the light spot 23, and which also causes the accuracy of the position control of the objective lens to be deteriorated.
The relationships between the respective detection currents Ia to Id and the converted voltages Va to Vd are EQU Va=Ia.multidot.Ra, Vb=Ib.multidot.Rb, Vc=Ic.multidot.Rc, and Vd=Id.multidot.Rd
wherein Ra to Rd represent the feedback resistors respectively installed in the operational amplifiers. In other words, the feedback resistors Ra to Rd correspond to the gain of each operational amplifier.
Assuming that the respective gains are equal to each other, i.e. the feedback resistors Ra to Rd have the same value of resistance (Ra=Rb=Rc=Rd=R), the X-direction output and Y-direction output indicating the degree of displacement of the light spot 23 are as follows.
When the center of the light spot 23 is on the central dividing point of the four-quadrant light receiving element 21, the amounts of light received by the respective quadrant light receiving elements 21a to 21d become mutually the same, Ia=Ib=Ic=Id. In this state, when the light spot 23 moves in the positive X direction, the following equation can be expressed: ##EQU1## wherein .DELTA.I indicates the change in the detection currents Ia to Id due to the changes in the light amounts received by the quadrant light receiving elements 21a to 21d.
In the above it is assumed that the feedback resistors Ra to Rd possess the same value of resistance, however after this the value of the resistance varies in each feedback resistor.
For instance, when the light spot 23 is displaced to the same degree as in the above case and the equations Ra=GR(G.noteq.1) and Rb=Rc=Rd=R are expressed, ##EQU2##
It is clear from the results of equation (1) and equation (2) that the sensitivity of the X direction output improves by (G+3)/4 times. However, an output from this kind of detector is normally corrected every time it is installed in a device, and therefore variations between devices do not raise a serious problem. Hence, fixed-value resistors are normally employed as feedback resistors.
The above explains the X-direction output detected when the light spot 23 shifts in the positive X direction. The following will discuss crosstalk, i.e. Y-direction output which becomes a spurious signal when the light spot 23 shifts in the X direction. When the equation, Ra=Rb=Rc=Rd=R, is expressed, the Y-direction output with respect to equation (1) is: ##EQU3## Namely, an output indicating the displacement in the Y direction, i.e. spurious signal, is not generated when the light spot 23 moves in the X direction.
In the meantime, when the value of resistance of the feedback resistors Ra to Rd varies from each other, Y-direction output with respect to equation (2) is given: ##EQU4## Hence, a spurious signal, that indicates as if the light spot 23 moves both in the Y direction and X direction when the light spot 23 actually moves in the X direction, is generated, i.e. crosstalk occurs.
The amount of crosstalk can be measured by the ratio of the output representing the true displacement (2) to the output of spurious-signal (4): ##EQU5## For example, when one of the feedback resistors has 1% error in resistance, the amount of crosstalk is calculated by substituting 1.01 for G in equation (5): about 0.25%.
Such a crosstalk will deteriorates the accuracy of the detection of the position of the objective lens and prevents the cancelation of spurious tracking error signal. In addition, when the mechanical characteristics of an optical disk, such as vibrations of the surface and eccentricity, are measured with the light spot position detector, the accuracy of measurement will be deteriorated due to crosstalk. Because the amount of crosstalk needs to be restrained to about 0.2% or less during the measurement of the mechanical characteristics. Thus the light spot position detector may not achieve the requirement with respect to crosstalk due to the accuracy of the feedback resistors Ra to Rd.